You are correct that it was minted in Potosi. Obviously predates the pillars and waves style so it is pre 1652. Looking closer below the P mintmark, it looks like a T assayer mark. The assayer is Juan Ximenez de Tapia. Unfortunately, he held the office over various time periods off and on. His last term of office as assayer was 1648. But looking at the devices of the Hapsburg Shield, the Austria element varies. Sometimes it is a clean thinner bar that intersects with the Flanders/Tirol and other times it is a thick block that seems to float. Based on the chunky block of the Austria element, my educated guess is that your piece was minted during the reign of King Philip IV between 1629-1639.